In known web printing presses, the web or webs of paper, after having been printed, may each be split into two or more ribbons by one or more longitudinal cutting devices. The ribbons thus cut may be superimposed with or without turning, and in a modifiable order with the aid of a device called a "reel of bars". Certain designs of reel of bars also allow finished webs or ribbons to be redirected in their run-off direction or allow their run-off axis to be offset.
A reel of bars comprises several turning bars or sets of turning bars. These bars are axisymmetric and cylindrical, and generally have a diameter varying between approximately 60 and 120 millimeters, and a length varying between 800 and 2200 millimeters. The turning bars are generally positioned horizontally, their longitudinal axis of symmetry making an angle of 45.degree. with the run-off direction of the paper upstream of the reel of bars. Thus, when a web or ribbon of paper arrives horizontally above a turning bar, tangential to its upper generatrix, the web or ribbon is partially wound around the turning bar and leaves it, tangential to its lower generatrix, still running off in a horizontal plane but now in a direction which is offset by 90.degree. with respect to its entrance direction.
In order to avoid any offset soiling on the web, which winds around the turning bar, an air cushion must be established between the bar and the web so that the web travels over the turning bar without touching it. For this purpose, a turning bar of the known type is a hollow bar which comprises a plurality of small holes called "blow holes" passing through the wall of the bar. These blow holes are distributed along several generatrices of the turning bar. The turning bar is supplied with pressurized air from an air propeller such as a blower or a centrifugal fan. The air supplied to the inside of the turning bar is expelled through the blow holes, which makes it possible to create an air cushion between the turning bar and the web.
It should be emphasized that since a web of paper has a variable width, and may or may not be cut into several longitudinal ribbons having variable widths, it does not cover all the blow holes and leaves a certain number of blow holes unused and uncovered and these are holes through which the air escapes, which reduces the pressure of the air cushion. The number of unused hole varies as a function of the width of the web or ribbon.
It is therefore necessary, when the web or ribbon passes through the turning bars, to plug the unused blow holes so as not to decrease the pressure of the air cushion.
A solution to this problem is to stick adhesive tape around the turning bar in the region of the unused holes for a specified width of ribbon, then to remove it or add some more when the width of the ribbon is increased or decreased.
Another known solution, described in EP 0 092 658, is to install, inside the turning bar, a plurality of liners, pierced with holes, which constitute rigid sleeves whose diameters are slightly less than the internal diameter of the turning bar. These liners can be moved translationally inside the turning bar, so as to occupy two positions. In the first position the holes of the liners coincide with the blow holes of the turning bar to allow the air to escape to the outside. In the second position the holes of the liners do not coincide with the blow holes so that the liners block the blow holes.
A drawback of this solution is that the translational displacement of the liners is not always precise which may lead to the unintentional blocking or freeing of certain blow holes and thus disturb the air cushion between the turning bar and the web or ribbon of paper.